Lead frames for optical semiconductor devices have been widely used in constitution parts of light sources for various display and lighting, in which light-emitting elements of optical semiconductor elements, such as LEDs (light-emitting diodes), are utilized as the light sources. Such an optical semiconductor device is produced by, for example, arranging a lead frame on a substrate, mounting a light-emitting element on the lead frame, and sealing the light-emitting element and its surrounding with a resin, to prevent deterioration of the light-emitting element and its surrounded region by external factors, such as heat, humidity, and oxidization.
When a LED is used as a light source for lighting, there is a demand for reflective materials for lead frames to have a high reflectance (e.g. reflectance 80% or more) in the whole regions of visible light wavelength (400 to 700 nm). Further, LEDs have been recently used as light sources for measurement/analytical equipments using ultraviolet rays, and there is a demand for the reflective materials to have a high reflectance in a wavelength of around 300 nm. Thus, in optical semiconductor devices to be used as light sources for lighting, the reflection property of reflective materials is a very important factor upon which product performance depends.
In order to cope with this demand, on a lead frame corresponding to the portion beneath a LED, there is often formed a layer (film) composed of silver or a silver alloy, for enhancing reflectance of light (hereinafter referred to reflectance). For example, known techniques include: forming of a silver plated layer formed on a reflection plane (Patent Literature 1); and forming a silver or silver alloy layer with grain diameter of 0.5 μm to 30 μm, by subjecting the layer to heat treatment at 200° C. or higher for 30 seconds or longer (Patent Literature 2).
However, as in the technique described in Patent Literature 1, in the case where the layer of silver or a silver alloy is simply formed on a lead frame, reflectance of light at a region of the vicinity of wavelength 300 nm to 400 nm corresponding to the near-ultraviolet region is conspicuously lowered. Furthermore, there is such a problem that migration is apt to occur in pure silver. On the other hand, as in the technique described in Patent Literature 2, in the case where the layer is formed to have grain diameter of 0.5 μm or more, the reflectance of light in the visible light region is somewhat improved. However, as is seen in the technique described in Patent Literature 1, reflectance is conspicuously lowered, with respect to light of the near-ultraviolet region.
Furthermore, the inventors of the present invention found that, when such a lead frame described in Patent Literatures 1 and 2 was utilized in an optical semiconductor device using a LED, luminance was deteriorated with the lapse of time. We further found that this was caused by a sulfur contamination contained even in a small amount in the resin for sealing the LED and the surrounding, which sulfur caused sulfuration of the silver due to the heat generated upon light emission from the LED, to change the color of the silver to black.
In order to solve this problem, there is a method of preventing sulfuration of silver in a film composed of silver or a silver alloy by applying a coating of various noble metals to the film. For example, a method was proposed, in which a palladium layer is formed in a thickness of 0.005 to 0.15 μm on a nickel underlying layer, and a rhodium layer is formed in a thickness of 0.003 to 0.05 μm as an outermost layer, to improve reflectance (Patent Literature 3). However, a lead frame formed by such a method is inferior to a lead frame with the film composed of silver or a silver alloy in reflectance, and it is difficult to achieve a level of 80% or more of reflectance which is required in a visible light region as a light source for lighting. Because of the presence of a portion whose reflectance is lower by 20% or more than that of a silver layer particularly in a rhodium layer, required characteristics of reflectance in blue-color or white-color optical semiconductor devices have not been satisfied.